Retroreflective sheets have found use in a wide range of applications, such as traffic signs, guide signs, warning signs, restriction signs, vehicle license plates, advertising signs, and so forth. An example of the retroreflective sheet is called an enclosed lens type including: a surface layer composed of at least one layer; a high-refractive-index glass bead; a focusing layer (also called a focusing resin layer); and a metal reflective layer, which are laminated in this order. Another configuration is what is called an encapsulated lens type including: a plurality of transparent spheres provided with a reflective mirror on its lower hemisphere; a resin support sheet for supporting the plurality of transparent spheres; and a transparent cover film that covers the plurality of transparent spheres by being disposed on the surface of the resin support sheet. A connection part for holding the cover film is formed on the resin support sheet. In the case of the encapsulated lens type, since the reflective mirror is formed directly on the surface of the transparent sphere, a reflective brightness at a small observation angle and up to a large incident angle is remarkably superior to that of the enclosed lens type, and thus the encapsulated lens type also is called a high-brightness retroreflective sheet. In the above-mentioned retroreflective sheet, a pressure-sensitive adhesive and a release paper or a release film further are laminated. Such a retroreflective sheet is attached onto a substrate, for example, a metal substrate, such as an aluminum plate, an iron plate, a coated iron plate and a stainless steel plate, or a plastic plate of fiber reinforced plastic (FRP), hard vinyl chloride or the like, and is used as a sign, a signboard or the like. In the daylight, such a retroreflective sheet is observed visually similarly to a typical sign or a typical signboard, but at night, the retroreflective sheet retroreflects the projected light regularly toward the light source direction, and thus has been useful for remarkably enhancing the visibility of the above-mentioned signs, vehicle license plates, signboards and the like. However, a hue of the conventional retroreflective sheet in daylight is substantially equal to a hue of light that is retroreflected toward the light source direction when the light is projected onto the conventional retroreflective sheet at night. Also, the worldwide standard of a retroreflective sheet requires that the hue in daylight is substantially equal to the hue of the retroreflected light when light is projected onto the retroreflective sheet at night.
Therefore, there has been a serious problem in that such a retroreflective sheet, which can create only monochromic colors whose hues do not differ between in daytime and at night even when being decorated or used for signboards, cannot satisfy the needs for variations and peculiarities of designs of the recent various products.
A retroreflective sheet in which such difficulty in the designability is solved and the decorativeness is added is suggested by the below-described Patent document 1. This retroreflective sheet is a laminate including: a layer of a combination of a transparent surface film and a glass sphere fixing layer; a glass sphere; a thin focusing layer; a metal reflective layer; a pressure-sensitive adhesive layer; and a release paper, in this order from the surface side. A rear surface of the layer of this combination of the surface film and the glass sphere fixing layer holds the glass sphere so that about a half of the glass sphere is embedded therein. And, a lower hemisphere of this glass sphere is covered with the thin focusing layer, and a metal reflective layer is evaporated on its rear surface and reflects a light beam, which is incident into the glass sphere from the surface side of the sheet, that is an upside thereof, toward the surface side of the sheet again. In this sheet, a scattering phenomenon of the light occurs when the light beam is incident into the glass sphere and is emitted therefrom, thereby obtaining a reflected light in rainbow colors. This retroreflective sheet has a problem in that it is not provided with an original reflective performance for obtaining such a reflected light in rainbow colors, but it actually is a sheet with high decorativeness. Such a sheet is called a rainbow film by those of ordinary skill in the art.
Also, a coating layer structure and a retroreflective material that are provided with retroreflective performance and decorativeness by using a retroreflective material and an optical interference material are suggested. The below-described Patent document 2 suggests a coating layer structure provided with a colored reflective layer that is formed on a predetermined substrate, the retroreflective material that is provided on a surface layer side of the colored reflective layer and a clear layer that is provided on a surface layer side of the retroreflective material, wherein glittering materials having transparency and regular reflectivity are arranged to be dispersed in the clear layer. Moreover, the below-described Patent document 3 suggests a coating layer structure provided with a light reflective layer that is formed on a predetermined substrate and a clear layer having a retroreflective material that is disposed on an upper layer side of the light reflective layer, wherein an optical interference material that is positioned on an upper layer side of the retroreflective material and has optical transparency and regular reflectivity is disposed in the clear layer. Furthermore, the below-described Patent document 4 suggests a colored light retroreflective material including a reflective substrate and transparent microspheres arranged to be arrayed on the substrate, wherein an interference material layer that provides an interference color is provided on the reflective substrate, which retroreflects colored light in a color different from the incident light toward an incident light entering direction.
Patent document 1: JP 5(1993)-45507 A
Patent document 2: JP 6(1994)-155693 A
Patent document 3: JP 7(1995)-128507 A
Patent document 4: JP 11(1999)-167010 A
However, regarding the rainbow sheet having the structure of Patent document 1, the color of the metal appears due to the presence of the metal reflective layer, and thus the color of the film becomes gray and lacks vividness. Moreover, rainbow colors that appear in daylight lack sharpness in hue, and do not provide attractiveness for the design using the rainbow sheet. Furthermore, when being used for wrapping vehicles and the like, the rainbow sheet has no retroreflective performance with respect to the irradiation of light by headlights of vehicles at night, and thus has a problem of lacking the designability and the visibility of the display that are the original purposes of the wrapping of the vehicles.
In addition, there is a problem in that, upon removing the metal reflective layer for avoiding the above-described influence of the metal, the reflectivity of light cannot be maintained and the rainbow colors disappear. Moreover, in the case of forming an information layer beneath the rainbow film so as to utilize it as a counterfeit preventing film, for applying the rainbow film to the counterfeit preventing film, the metal reflective layer reflects light substantially perfectly, and thus the displaying of the information layer positioned beneath the metal reflective layer cannot be observed visually. On the other hand, in the case of forming the information layer above the metal reflective layer so as to utilize it as the counterfeit preventing film, light passes through the above-described information layer before passing through the surface film and the glass spheres and emitting, and thus there is an inevitable problem in that the presence of the information layer is observed visually in general diffused light. As a result, those of ordinary skill in the art presently lose hope for obtaining a sheet that adopts the retroreflective sheet structure, has high decorativeness, and provides sufficient function as the counterfeit preventing film, in spite of the strong demands in the market.
Moreover, since the optical interference materials of Patent documents 2 and 3 uses mica as core materials, each of them uses a material that performs regular reflection with respect to a part of the light with a certain wavelength, and allows residual light to pass through as transmission light. However, each of these optical interference materials has a glittering appearance, but has a problem in that reflected light from the core material is decreased and thus an interference light amount is decreased, whereby a hue change in the diffused light is not sufficient, and the hue change by the regularly reflected light is also insufficient even in the projection of light at night. Furthermore, since the interference material layer of the above-described Patent document 4 is provided beneath the transparent microspheres, the interference light mainly depends on the retroreflected light and colors only the retroreflected light that are passed through the transparent microspheres, and the colors cannot be observed visually in diffused light from various directions in dependence on a point of view. In particular, in the projection of light at night, the interference material layer usually colors only retroreflected light, and thus has a problem in that the hue change of multicolor cannot be sufficiently observed depending on the viewing direction.